Cassini Looks at Saturn’s Rings, Gets a Bonus Planet

Bad Astronomy
The entire universe in blog form
May 5 2014 12:16 PM

Cassini Spies Another Pale Blue Dot

Uranus and Saturn
The long view: Cassini looks past Saturn's rings to see Uranus, billions of kilometers away.

Photo by NASA/JPL-Caltech/SSI

Speaking of Saturn’s rings, on April 11, 2014, Cassini was taking a snapshot of Saturn’s thin outer F ring when it got a bonus: Uranus!

The picture above shows the seventh planet from the Sun as a small, pale blue dot (hmmmm, that sounds familiar). Uranus is actually a big planet, four times wider than Earth, but it was a staggering 4.3 billion kilometers (2.7 billion miles) from Saturn when this shot was taken, nearly on the opposite side of the Sun. It’s amazing Cassini saw anything at all (though, to be honest, in the image Uranus was brightened by a factor of 4.5 to make it more obvious, and the A ring—the broad ring at the lower right—was also brightened for clarity). Uranus was just barely more than a dot, even to Cassini’s narrow-angle camera.

Zooming in on Uranus.

Photo by NASA/JPL-Caltech/SSI


Uranus is generally visible from Earth using binoculars; it’s just on the edge of naked-eye visibility. Right now it’s an early-morning object near Venus in the predawn sky. Seeing it through even a small telescope is a bit shocking; it’s clearly a disk, not a point-like dot like a star, and a very distinctive blue-green. The upper atmosphere of the planet has a lot of methane which is excellent at absorbing red light. Sunlight that hits Uranus gets its red removed, leaving the blues and greens to reflect off and travel their way back to us.

With bigger telescopes, Uranus has another surprise: rings! You can’t see them in the Cassini shot—they’re far too faint—but they’re there. So we’re looking through the rings of one planet to spy on another that has rings as well. The solar system is a weird place.

Another funny thing that struck me in this image is how well-behaved the F ring looks! Usually it’s a mess, but it depends on what part you’re looking at. The moons Prometheus and Pandora can stir things up, for example, and sometimes an impact with a meteoroid can create havoc as well. It’s actually rather odd to see the F ring so well-groomed here (although, if you look carefully, you can see some twists and changes in it as your eye seeps around it).

It’s fun to see images like this, analyze them, and see what surprises lie therein. You might see an entirely other world … or you might have to just be satisfied with seeing the splendiferous glory of Saturn’s rings dancing to the tune of Newtonian gravity. I’ll take it.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


The Slatest

Ben Bradlee Dead at 93

The legendary Washington Post editor presided over the paper’s Watergate coverage.

This Scene From All The President’s Men Captures Ben Bradlee’s Genius

Renée Zellweger’s New Face Is Too Real

Sleater-Kinney Was Once America’s Best Rock Band

Can it be again?

Whole Foods Is Desperate for Customers to Feel Warm and Fuzzy Again

The XX Factor

I’m 25. I Have $250.03.

My doctors want me to freeze my eggs.

The XX Factor
Oct. 20 2014 6:17 PM I’m 25. I Have $250.03. My doctors want me to freeze my eggs.

Forget Oculus Rift

This $25 cardboard box turns your phone into an incredibly fun virtual reality experience.

George Tiller’s Murderer Threatens Another Abortion Provider, Claims Free Speech

The Congressional Republican Digging Through Scientists’ Grant Proposals

  News & Politics
The World
Oct. 21 2014 3:13 PM Why Countries Make Human Rights Pledges They Have No Intention of Honoring
Oct. 21 2014 5:57 PM Soda and Fries Have Lost Their Charm for Both Consumers and Investors
The Vault
Oct. 21 2014 2:23 PM A Data-Packed Map of American Immigration in 1903
  Double X
The XX Factor
Oct. 21 2014 3:03 PM Renée Zellweger’s New Face Is Too Real
  Slate Plus
Behind the Scenes
Oct. 21 2014 1:02 PM Where Are Slate Plus Members From? This Weird Cartogram Explains. A weird-looking cartogram of Slate Plus memberships by state.
Brow Beat
Oct. 21 2014 9:42 PM The All The President’s Men Scene That Perfectly Captured Ben Bradlee’s Genius
Oct. 21 2014 5:38 PM Justified Paranoia Citizenfour offers a look into the mind of Edward Snowden.
  Health & Science
Climate Desk
Oct. 21 2014 11:53 AM Taking Research for Granted Texas Republican Lamar Smith continues his crusade against independence in science.
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.